Child maltreatment appears to be the single most preventable cause of mental illness and behavioral dysfunction in the United States. Few published studies examine the developmental and the psychobiological consequences of sexual abuse. There are multiple mechanisms through which sexual abuse can cause post-traumatic stress disorder, activate biological stress response systems, and contribute to adverse brain development. This article will critically review the psychiatric problems associated with maltreatment and the emerging biologic stress system research with a special emphasis on what is known about victimization by sexual abuse. (Contains 2 figures.)